Many products and procedures have been used, or proposed for use, in order to change or modify teeth in the human mouth. In orthodontic procedures, the objective is to change the relative positions of two or more teeth. In some cosmetic procedures, the objective is to change the perceived color of teeth so that the teeth appear whiter.
One orthodontic procedure makes use of one or more rigid polymeric shells each of which fits over the teeth and, when in place, does not fit precisely to one or more of the teeth and, therefore, imparts a displacing force to the teeth and over the course of time changes the relative positions of two or more teeth. Reference may be made for example to U.S. Pat. No. 5,975,893 (Chishti) and U.S. Pat. No. 7,220,122 (Chishti), the entire disclosure of each of which is incorporated herein by reference. Many polymers, including some of those used in dental and orthodontic applications, have a slightly yellow color which is produced during manufacturing or processing such as extrusion, injection molding and/or thermoforming. Consequently, although the primary objective of the rigid polymeric shell is functional, it often contains a small quantity of additives to change the yellow color. This is referred to as “color compensation” or “color correction”.
Much effort and expense has been expended in order to whiten teeth. However, the known cosmetic procedures have substantial disadvantages. Many cosmetic procedures make use of bleaching agents which are applied directly to the teeth; bleaching agents can damage the natural tooth structure itself and/or can produce different cosmetic effects on the natural tooth structure and/or stains and/or fillings and/or crowns. Other cosmetic procedures make use of compositions which form a coating on top of the teeth; those compositions are difficult to apply uniformly and accurately, and degrade relatively rapidly and/or are difficult to remove. For information about known cosmetic procedures, reference may be made for example to U.S. Pat. Nos. 3,986,261, 4,032,627, 5,716,208, 6,343,933, 6,368,576, 6,503,485, 6,986,883 and 7,214,262, and US Patent Publications 2005/0175552 and 2007/0086960, the entire disclosure of each of which is Incorporated herein by reference for all purposes